Geological Survey Bulletin 612Guidebook of the Western United States: Part B

PRINCIPAL DIVISIONS OF GEOLOGIC TIMEa[A glossary of geologic terms is given on pp. 182-85.]

Era.

Period.

Epoch.

Characteristic life.

Duration according to various estimates.

Millions of years.

Cenozoic (recent life).

Quaternary.

Recent.Pleistocene(Great Ice Age).

"Age of man." Animals and plants of modern types.

1 to 5.

Tertiary.

Pliocene.Miocene.Oligocene.Eocene.

"Age of mammals." Possible first appearance of man. Rise and
development of highest orders of plants.

Mesozoic (intermediate life).

Cretaceous.

(b)

"Age of reptiles." Rise and culmination of huge land reptiles
(dinosaurs), of shellfish with complexly partitioned coiled shells
(ammonites), and of great flying reptiles. First appearance (in Jurassic)
of birds and mammals; of cycads, an order of palmlike plants (in
Triassic), and of angiospermous plants, among which are palms and
hardwood trees (in Cretaceous).

"Age of fishes." Shellfish (mollusks) also abundant. Rise of amphibians
and land plants.

Silurian.

(b)

Shell-forming sea animals dominant, especially those related to the
nautilus (cephalopods). Rise and culmination of the marine animals
sometimes known as sea lilies (crinoids) and of giant scorpion-like
crustaceans (eurypterids). Rise of fishes and of reef-building
corals.

Ordovician.

(b)

Shell-forming sea animals especially cephalopods and mollusk-like
brachiopods, abundant. Culmination of the buglike marine crustaceans
known as trilobites. First trace of insect life.

Cambrian.

(b)

Trilobites and brachiopods most characteristic animals. Seaweeds (algae)
abundant. No trace of land animals found.

Proterozoic(primordial life).

Algonkian.

(b)

First life that has left distinct record. Crustaceans, brachiopods, and
seaweeds.

Archean.

Crystallinerocks.

No fossils found.

50+.

aThe geologic record consists of sedimentary
bedsbeds deposited in water. Over large areas long periods of
uplift and erosion intervened between periods of deposition. Every such
interruption in deposition in any area produces there what geologists
term an unconformity. Many of the time divisions shown above are
separated by such unconformitiesthat is, the dividing lines in the
table represent local or widespread uplifts or depressions of the
earth's surface.bEpoch names omitted; in less common use than
those given.